In this hypothetical, the AI would be trained on fully clothed adults and children. As well as what many of those same adults look like unclothed. It might not get things completely right on its initial attempt, but with some minor prompting it should be able to get pretty close. That said, the AI will know the correct head size proportions from just the clothed datasets. It could probably even infer limb proportions from the clothed datasets as well.
This is the part of the conversation where I have to admit that you could be right, but I don’t know enough to say one way or the other. And since I have no plans to become a pediatrician, I don’t intend to go find out.